This invention relates to the field of bearings for internal combustion engine connecting rod.
As is known in the art internal combustion engines are generally constructed in the form of a piston within a cylinder to form a combustion chamber. The piston is in turn connected through a pivoted connecting rod which is pivoted at an upper end to a pin in the piston and a lower end is connected to an offset crank shaft rod journal.
The entire force of the engine is exerted through the connecting rod in tension and compression against the crankshaft to be converted into rotary engine motion. In order to reduce friction and resist the loads, a bearing is then placed within the lower end of the connecting rod, where it connects to the crankshaft. This bearing is lubricated with pressurized oil through the crankshaft through internally provided oil passages within the bearing, and the connecting rod in fact rides and pivots upon a thin film of oil as the bearing is an otherwise high tolerance or tight fit, required to avoid vibration, impact and slope and loss of power in transferring the reciprocating motion of the piston into the rotary motion of the crankshaft.
In order for the piston to function properly within the cylinder, it is known that the piston is provided with certain compressed piston rings lubricated by oil which is slung into the pistons and on the cylinder walls by centrifugal force by rotation of the crankshaft rod throw as oil is forced from between the rod journal and the rod bearing. This oil also provides a gas tight seal permitting the development of adequate compression, efficient burning, and transfer of the force generated by expanding burned gas to the piston. These piston ring seals are vital to the successful operation of a reciprocating engine and are deliberately designed to absorb the frictional wear and the reciprocating motion of the piston within the cylinder, as they may be readily replaced.
However, operation of the piston within the cylinder tends to wear the cylinder walls where the piston rings travel. Thus, after a period of usage of the engine, it will be found that the cylinder walls have been worn by the piston rings to a slightly greater inside diameter than the original inside diameter of the cylinder. In effect, this forms a small but significant ridge within the cylinder extending from immediately above the upper travel of the upper piston ring when the piston is at top dead center to the top of the cylinder wall within the engine block.
When an engine is rebuilt to extend its life, the wearable bearing components are replaced and this includes the connecting rod and crankshaft bearings as well as the piston rings. In reassembling the engine, the piston is removed and inserted through the top of the cylinder head using various well known tools to compress the piston rings around the piston and allow the piston to be slid into the cylinder. However, operation of the engine would result in impact of the replaced piston ring against the ridge that has been formed by the previous wear of the piston rings within the cylinder. This impact in turn would damage or break the top ring and bend or break the land on the piston and cause ultimately catastrophic failure of the engine. In order to prevent this, it is therefore necessary to ream the ridge out of the cylinder, typically by the use of a ridge reamer. Unfortunately, the cylinder as originally constructed in the engine block is provided with a slight taper at its upper end so as to permit easy entry of the piston rings when inserted. Reaming out the ridge provides a sharp corner at the upper end of the cylinder wall in the engine block and this in turn can result in chipping and breakage of piston rings which is undetected by the repair person during insertion and can lead subsequently to failure.
The reaming process also requires special tools and a considerable amount of skill in order to avoid reaming the ridge to other than a perfect cylindrical shape which must be maintained to a high tolerance in order that the replaced piston rings will form a gas tight seal.
If the cylinder is excessively reamed below the ridge, gas leakage or blowby will occur. If the reaming does not maintain a high tolerance cylindrical shape, the top ring could wedge and fracture the piston. Insufficient reaming, on the other hand, may permit the top ring to make contact with a portion of the ridge not removed, which could also cause the top ring to wedge or fracture the piston.